Shafts forming a power transmission system of an automobile include a drive shaft for connecting a final reduction gear and a wheel bearing device to each other, and a propeller shaft for transmitting a power from a transmission to a final reduction gear. At a shaft end portion of each of the shafts, a spline or the like serving as a coupling element is provided. In terms of basic structure, intermediate shafts for the drive shaft are roughly classified into a solid shaft fabricated from a solid bar stock, and a hollow shaft fabricated from a bar stock or a steel tube by plastic working and a joining process.
Conventionally, the solid shaft has been normally used. However, in recent years, there is a need for functional improvements such as a reduction in weight of a suspension system of an automobile and an improvement of torsional rigidity and NVH characteristics, and hence the hollow shaft is being increasingly used.
As an integral type hollow shaft, for example, there are used shafts molded by swaging that is a process of reducing a diameter of a steel tube by hammering in a diameter direction at high speed while rotating the steel tube about an axis thereof, or by press working that is a process of reducing the diameter of the steel tube by pushing the steel tube into a die in an axial direction thereof. In the integral type hollow shaft thus fabricated by plastic working such as swaging, a blank plastically flows radially inward when the steel tube is reduced in diameter, and hence working wrinkles are likely to remain in the radially inner surface (inner surface). The working wrinkles in the radially inner surface may act as a starting point of a fracture when torsional torque is applied to the shaft. Thus, there is a fear in that, when this region starts to fracture, a static torsional strength and a torsional fatigue strength are lowered.
In this context, conventionally, various hollow shafts increased in strength have been proposed (Patent Literature 1 to Patent Literature 3). Patent Literature 1 proposes the following shaft. Specifically, the shaft has the structure that the opening end surface of the cylindrical portion of the shaft forming member is joined to the opening end surface of another shaft forming member, thereby increasing the joining area. This structure increases the strength of the joining portion. Patent Literature 2proposes the following shaft. Specifically, the part of the shaft, in which the spline is to be provided, is molded by plastic working before being subjected to splining. The inner diameter of the part after the molding is smaller than the inner diameter of the part before the molding, and the thickness of the part is substantially equal before and after the molding. Patent Literature 3 proposes the shaft in which wrinkles resulting from plastic working are removed by cutting and grinding before carrying out quench hardening treatment.